1. Field of the Invention
The present invention relates to a motor for an electric power steering apparatus in which a wave washer for applying a preload to a rolling element bearing, which supports a shaft of a rotor for rotation relative thereto, is arranged between the rolling element bearing and a bearing box in which the rolling element bearing is received.
2. Description of the Related Art
As a conventional motor, there has been known one in which a casing with a rotor received therein is formed with a bearing box in which a rolling element bearing for rotatably supporting a shaft of the rotor is received, and a wave washer for applying a preload to an outer race of the rolling element bearing is arranged between a bottom surface of the bearing box and the rolling element bearing (see, for example, a first patent document: Japanese patent application laid-open No. 2002-359945).
In case where such a motor is used with an electric power steering apparatus, it is possible to suppress noise resulting from vibrations of the outer race of the rolling element bearing in the diametrical, circumferential and axial directions thereof by applying a biasing force in the axial direction to the outer race under the action of a resilient force of the wave washer.
In this case, however, when the preload of the wave washer exceeds an appropriate value, a friction between the outer race of the bearing and rolling elements such as balls, rollers, or the like and a friction between the rolling elements and an inner race of the bearing are increased, so a loss torque of the motor accordingly becomes large. Therefore, the rotation following capability of a steering wheel during the steering operation thereof is deteriorated, and the driver comes to feel that the steering wheel is heavy.
Accordingly, in case where the above-mentioned motor is used with the electric power steering apparatus, it is necessary to control the preload of the wave washer in a strict manner in order to satisfy the effect of noise control and to suppress the amount of increase of the loss torque to within an appropriate range.
The preload of the wave washer is decided by the spring constant of the wave washer itself and the compressed height thereof at the time when the wave washer is built into the bearing box, but in case where an axial dimension between the bottom surface of the bearing box and the rolling element bearing changes, the compressed height of the wave washer is varied, so it is necessary to change the spring constant of the wave washer so as to provide an appropriate preload.
Therefore, there has been a problem that it is necessary to provide a new wave washer which is changed in its height or thickness.